External fixation system

ABSTRACT

An external fixation system comprising: first and second planar at least part-circular ring elements, the first ring element having a circumferential track extending along the part-circular circumference thereof; a plurality of struts each having a first and second end, the first end of each strut coupled to the first ring by a first connector and the second end of each strut coupled to a second ring by a second connector, the first connector including a spherical joint; the second connector non-rotatably coupled to the second ring, the strut second end being coupled to the second connector by a U-joint; shuttles mounted on the track of the first ring for movement there along with one shuttle coupled to each strut; and means for controlling the angular position of each strut second end and means for controlling the position of each shuttle along the circumferential track on the first ring.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 61/209,677 filed Mar. 10, 2009, theentire disclosure of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The external fixation market can be divided into two major segments:acute trauma and reconstructive. The customers, products, and needs ofeach segment are distinctly different. The trauma segment is dominatedby modular fixators. These frames are characterized by limitedcomponentry and very rapid application. Consequently, they are known forbeing fairly simple products. Most of these frames are used fortemporizing fixation and quite often are only on the patient for hoursor days.

The reconstructive segment leans heavily toward ring fixation, butunilateral frames also enjoy an appreciable market share. Ring fixatorssuch as the well known Ilizarov frame are by far the most stable andcapable of external fixators. Such frames are shown in U.S. Pat. Nos.4,365,624; 4,615,338; 4,978,348; 5,702,389; and 5,971,984. Their use ofa combination of pins and wires to achieve a variety of polyaxialpin/wire attachments creates this stability. They can accomplish a fullsix axes of deformity correction and, when applied and managed well, cancorrect primary deformities while not creating secondary deformities.Rotational deformities are the sole domain of the ring fixator. However,mastery of the techniques and the products themselves is a long anddaunting process that it is not attractive to many users.

The present invention relates to a method for using an improvedorthopaedic external fixator including a mechanism that allows two boneelements or portions to be fixed relative to one another while allowingcomplete repositioning of the two bone elements or portions relative toone another.

It is often necessary to realign, reposition and/or securely hold twobone elements relative to one another. For example, in the practice ofmedicine, bone fragments and the like must sometimes be aligned orrealigned and repositioned to restore boney continuity and skeletalfunction. At times, this may be accomplished by sudden maneuver, usuallyfollowed by skeletal stabilization with cast, plate and screws,intramedullary devices, or external skeletal fixators.

A bone fragment can be moved, in general, from its original position asin a nonunion or malunion or from its intended position as in congenitaldeformities along six separate axes, a combination of three orthogonaltranslational axes (e.g., typical “X,” “Y” and “Z” axes) and threeorthogonal rotational axes (e.g., rotation about such typical “X,” “Y”and “Z” axes).

External fixation devices are attached to the boney skeleton withthreaded and/or smooth pins and/or threaded and/or smooth and/or beadedwires. Such constructs are commonly referred to as orthopaedic externalfixators or external skeletal fixators. External fixators may beutilized to treat acute fractures of the skeleton, soft tissue injuries,delayed union of the skeleton when bones are slow to heal, nonunion ofthe skeleton when bones have not healed, malunion whereby broken orfractures bones have healed in a malposition, congenital deformitieswhereby bones develop a malposition, and bone lengthening, widening, ortwisting.

A circumferential external fixator system was disclosed by G. A.Ilizarov during the early 1950s. The Ilizarov system includes at leasttwo rings or “halos” that encircle a patient's body member (e.g., apatient's leg), connecting rods extending between the two rings,transfixion pins that extend through the patient's boney structure, andconnectors for connecting the transfixion pins to the rings. Use of theIlizarov system to deal with angulation, translation and rotation isdisclosed in “Basic Ilizarov Techniques,” Techniques in Orthopaedics®,Vol. 5, No. 4, December 1990, pp. 55-59.

Prior art orthopaedic external fixators differ in their ability to moveor adjust one bone fragment with respect to the other in a gradualfashion. Some allow gradual translation, others allow gradual rotationabout two axes. The Ilizarov system can provide an external fixationdevice that could provide gradual correction along and about six axes;however, such a device would require many parts and would be relativelycomplicated to build and use in a clinical situation.

Often orthopaedic external fixators such as Ilizarov fixators must berepositioned after their initial application. Such modification may benecessary to convert from one correctional axis to another or to convertfrom an initial adjustment type of fixator to a weight bearing type offixator, some of the correctional configurations not being stable enoughfor weight bearing.

A “Steward platform” is a fully parallel mechanism used in flight andautomotive simulators, robotic end-effectors, and other applicationsrequiring spatial mechanisms with high structural stiffness and includesa base platform, a top platform, and six variable limbs extendingbetween the base and top platforms. See S. V. Sreenivasan et al.,“Closed-Form Direct Displacement Analysis of a 6-6 Stewart Platform,”Mech. Mach. Theory, Vol. 29, No. 6, pp. 855-864, 1994.

Taylor et al. U.S. Pat. No. 5,702,389 relates to a fixator that can beadjusted in six axes by changing strut lengths only, without requiringjoints to be unclamped, etc. This patent includes a first ring member orswash plate for attachment relative to a first bone element; a secondring member or swash plate for attachment relative to a second boneelement. Six adjustable length struts having first ends movably attachedto the first member and second ends movably attached to the secondmember are provided. The first ends of the first and second struts arejoined relative to one another so that movement of the first end of oneof the first and second struts will cause a corresponding movement ofthe first end of the other strut, with the first ends of the third andfourth struts joined relative to one another so that movement of thefirst end of one of the third and fourth struts will cause acorresponding movement of the first end of the other strut. The thirdand fourth struts and fifth and sixth struts are similarly joined.Second ends of the first and sixth struts joined relative to one anotherso that movement of the second end of one of the first and sixth strutswill cause a corresponding movement of the second end of the otherstrut. Second ends of the second and third struts and fourth and fifthstruts are formed in a similar manner. Thus, changing the length of thestruts effects reposition of the bone segments.

BRIEF SUMMARY OF THE INVENTION

A parallel robot is defined as a manipulator consisting of a fixed baseand an end-effector with 6 degrees of freedom (DOF) that are linkedtogether by at least two independent kinematic chains. Actuation of sucha device takes place through 6 simple actuators. It is important to notethat the number of actuators is equal to the number of degrees offreedom; a 6 DOF robot will require six actuators. Furthermore, eachconnecting chain must also have 6 degrees of freedom. Each DOF comesfrom a joint connecting two rigid bodies within the chain. The mostcommonly used joints in parallel robots are revolute (R), prismatic (P),universal (U), and spherical (S). R and P joints each grant one DOF, Ujoints grant two, and S joints give three. Universal joints consist oftwo revolute joints whose axes of rotation intersect, and are sometimestreated as two joints instead of one.

The general nomenclature for describing a parallel robot's configurationis to list the number of struts followed by the joint setup, withactuated joints underlined. Our present device is of a 3-USR (or 3-RRSR)configuration. The Base Adjustment Unit (BAU) is a modified U-joint; oneof the axes of rotation is controlled through the worm-gear interface,and the other is free. The strut connects the BAU to the sliding unitvia a free spherical joint, and the sliding unit revolves around theupper ring by another worm-gear interface. This leads to a kinematicchain with 2 DOF×3 chains=6 DOF.

The design shown in provisional application No. 61/209,677 filed Mar.10, 2009 has 6 DOF while using only three struts. However, it has analternate joint configuration. The BAU was connected to the second ringvia an assembly that allowed it to freely swivel, and the strut'sconnection to the sliding unit was via free universal joint instead of aspherical joint. This led to a 3-RUUR (or 3-RRRRRR) configuration.Again, both configurations satisfy the necessary conditions to beconsidered a parallel robot.

The proximal U-joints of provisional application No. 61/209,677 arereplaced with a ball and socket (or spherical joint) and the baseadjustment unit (BAU) is no longer free to spin about the axis thatconnects it to the second ring.

An embodiment of the presently disclosed external fixation system hasfirst and second planar at least part-circular ring elements. Thecenters of the first and second ring elements are spaced along an axis.The first ring element has a circumferential track extending along thepart-circular circumference thereof. Three variable length struts eachhaving first and second ends are provided. The variable length strutscan be locked at a desired length after the initial positioning of thering element. The first end of each strut is coupled by a firstconnector to the first ring and the second end of each strut is coupledto a second ring by a second connector. The second connector strut isfixedly coupled to the second ring in a manner which prevents rotationof the connector about an axis perpendicular to the plane of the secondring. The second connector has a U-joint having a first axis allowingfor rotation in a plane parallel to the plane of the second ring. TheU-joint has a second axis allowing rotation about the first axis. Threeshuttles are mounted on the track of the first ring for movement therealong with one shuttle coupled to the first end of each strut via thefirst connector. Means are provided for controlling the rotational andangular position of each strut second end and means for controlling theposition of each shuttle along the circumferential track on the firstring.

The first and second rings may be complete or half circles or may beother geometric shapes, such as square or rectangular. The shuttleconnected to the strut first ends may be spaced 120° around acircumference of the first ring. However, the three shuttles mounted onthe first ring are movable and can move along an arc limited only by theposition of the adjacent shuttle. Thus, the first ends of the threestruts may move through a large portion of the first ring circumference.

Preferably, each shuttle or sliding unit can move along the track on thefirst ring in, for example, one to five degree increments and theangular location of the strut second end with respect to the second ringmay be in five to ten degree increments. In this case, there would bebetween 36 and 72 holes spaced equally around the lower ring formounting the second connector. The angular movement of the shuttle withrespect to the first ring can also be infinitely variable.

Each strut first end has a connector with a spherical joint coupling thestrut to the shuttle mounted on the first ring and a connector with astandard U-joint coupling each strut second end to the second ring. TheU-joint has a drive system controlling the movement of the joint aboutone axis of the U-joint. Preferably, the drive axis is parallel to theplane of the second ring. The drive system preferably comprises acomputer controlled stepper or servo motor and a gear drive system.

The external fixation system has first and second ring elements thefirst ring having three shuttles mounted on the ring element forcontrolled movement about a circumference of the ring. There are threeconnectors fixed to the second ring with three struts having first endsconnected to a respective shuttle by connectors having a spherical orball joint and a second end connected to a respective connector on thesecond ring by a standard U-joint rotatable about a first and secondaxis and rotatable in a controlled manner about the second axis which isperpendicular to the first axis. The first axis preferably extendsparallel to the plane of the ring and the connector is fixed on the ringin a manner to prevent its rotation about the mounting hole axis. Aprogrammable or microprocessor controller is provided for controllingthe movement of the shuttle about the circumference of the first ringand for controlling the movement of the strut second end about the firstaxis of the connector on the second ring. This can also be adjustedmanually.

An embodiment of the presently disclosed external fixation systemincorporates three variable length struts that can be adjusted in lengthand then locked. Once locked the struts can manipulate the relativeposition of bone fragments to one another. The system is capable ofmoving in six degrees of freedom (DOF). For movement, it willincorporate either the calibration device disclosed in U.S. Pat. No.6,017,534, the entire disclosure of which is incorporated herein byreference, or six dedicated servo or stepper motors respectively coupledto the shuttles and second ends of each strut. The calibration device orservo/stepper motors are controlled by software. The software is theinterface the surgeon or user will use to determine the dailyadjustments of the frame assembly. The system incorporates positionsensors such as potentiometers and/or optical encoders and/or otherposition sensors at the moving points along the frame to not onlydetermine the initial position, for software input/setup, but also toprovide feedback to insure that the daily adjustments are being madeproperly.

An embodiment of the presently disclosed external fixation system hastwo rings and three fixable length struts having a spherical joints on afirst connector on the first ring and a standard U-joints having twoaxes of rotation. The U-joint has a first of the two axes of rotationcontrolled (cannot move freely) by a worm and worm gear. The other axisof the second connector moves freely about an axis perpendicular to thefirst axis.

The movement of the fixed strut about the first axis and third axis isfree moving. The movement about the second axis is controlled by theinteraction of the worm gear and a worm situated on the second ringwhich worm extend parallel to the worm gear.

The driving connection of the worm to the “smart tool” described in U.S.Pat. No. 6,017,354 or servo/stepper motor will be a miter and bevelconnection (where the worm has a bevel gear at one end and thetool/motor has a miter gear at its respective end). This relationshipwill allow the motor's miter gear to drive the worm's bevel gear whichin turn drives the worm to the worm gear. This action controls one ofthe axes of rotation that affects its respective strut's angle (relativeto the second ring).

The first connector couples the struts to the first ring by a slidingshuttle unit, which glides along two circumferential grooves on eitherside of the first ring. The grooves are spaced radially inwardly thetoothed outer circumferential surface of the first ring. The movement ofthe sliding unit is controlled by the interaction of worm gear teeth (onthe outside of the first ring) and a worm. Each sliding unit can bemoved about the circumference of the moving ring independently. The wormis to be driven by a “smart tool” or a dedicated stepper/servo motor ora hand tool. The driving connection of the worm to the tool or motor ispreferably a miter and bevel gear connection where the worm has a bevelgear at one end and the smart tool/motor has a miter gear at itsrespective end. Any gear box and motor could be used to drive thesliding units.

With six points of adjustment, the system will have complete control (insix axes of rotation) over the relative position of the two ringswithout changing strut length.

In an alternate embodiment, the external fixation system includes afirst platform, a second platform, and a plurality of non-prismatickinematic chains. By “non-prismatic,” it is meant that the kinematicchain links do not extend in length during actuation. Each kinematicchain connects the first platform to the second platform and includes atleast two actuated joints. At least one actuated joints is configured tomove along a perimeter of the first platform. This embodiment furtherincludes a means for actuating the actuated joints.

As used herein when referring to bones or other parts of the body, theterm “proximal” means close to the heart and the term “distal” meansmore distant from the heart. The term “inferior” means toward the feetand the term “superior” means toward the head. The term “anterior” meanstoward the front part or the face and the term “posterior” means towardthe back of the body. The term “medial” means toward the midline of thebody and the term “lateral” means away from the midline of the body.

While the system has been described for use in an external fixation ringsystem the identical structures and principles could be used in anyapplication where platforms are manipulated such as a Stewart platform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the external fixation ring used in theexternal fixation system of the present invention;

FIG. 2 is an isometric view of a pair of rings shown connected by threestruts;

FIG. 3A is an isometric view of an actuated joint connected to the ring;

FIG. 3B is an isometric cross-sectional view of the actuated jointconnected to the ring;

FIG. 3C is an elevation cross-sectional view of the actuated jointconnected to the ring;

FIG. 4A is an isometric view of a shuttle or sliding unit mounted on theupper ring of the present invention;

FIG. 4B is an elevation view of the shaft and universal joint andconnector of 4A;

FIG. 4C through FIG. 4E are sequential views of the mounting of theshuttle or sliding unit of the present invention on the upper ring shownin FIG. 4A;

FIG. 4F shows a drive system for the shuttle or sliding unit of thepresent invention when mounted on the ring of FIG. 4A;

FIG. 4G through FIG. 4J show the worm drive contained in shuttle orsliding unit in a disengaged position and an engaged position with theteeth on the outer circumference of the ring of FIG. 4A;

FIG. 5 is a depiction of an x, y, z coordinate system including variouspoints of the second ring where struts are attached thereto;

FIG. 6 is a plurality of spherical surfaces showing the movement of theend of one of the three struts shown in FIG. 1;

FIG. 7 is a geometric representation of the strut and attached to thefirst upper ring at its upper end to the second lower ring at itsorigin;

FIG. 8 is an isometric view of an alternate embodiment of the externalfixation system of the present invention;

FIG. 9 is an elevation view of a first strut portion and a ringconnector of the alternate embodiment of the present invention;

FIG. 10 is a cross-sectional view of the strut element of FIG. 9 alonglines 10-10;

FIG. 11 is an elevation view of the strut;

FIG. 12 is an isometric view of the strut end of FIG. 11;

FIG. 13 is an elevation view of the extension mechanism of the strut ofthe alternate embodiment of FIG. 8;

FIG. 14 is an isometric view of an alternate embodiment of the externalfixation system of the present invention;

FIG. 15 is an isometric view of a ring or platform of the externalfixation system shown in FIG. 14;

FIGS. 16 and 17 are isometric views of a kinematic chain of the externalfixation system shown in FIG. 14;

FIG. 18 is cross-sectional view of the kinematic chain shown in FIGS. 16and 17;

FIG. 19 is an exploded cross-sectional view of the kinematic chaindepicted in FIGS. 16 and 17;

FIG. 20 is an isometric view of a gear portion, a yoke, and a pin of thekinematic chain shown in FIGS. 16 and 17;

FIG. 21 is an exploded view of the gear portion, the yoke, and the pindepicted in FIG. 20;

FIG. 22 is an isometric view of a sliding unit of the external fixationsystem shown in FIG. 14;

FIG. 23 is an exploded view of the sliding unit shown in FIG. 22;

FIG. 24 is an isometric view of a measuring device attached to theembodiment of the external fixation system shown in FIG. 14;

FIG. 25 is an isometric view of a measuring device for determining thedistance between the osteotmy site and a ring or platform of theexternal fixation system;

FIG. 26 is an isometric view of a six strut external fixation systemaccording to an embodiment of the present disclosure;

FIG. 27 is another isometric view of the six strut external fixationsystem shown in FIG. 1;

FIG. 28 is a side view of a shuttle unit connected to an upper ring;

FIG. 29 is a side view of the external fixation system shown in FIG. 26;

FIG. 30 is an isometric view of the external fixation system of FIG. 26,showing the lower and upper rings oriented in a first angular position;

FIG. 31 is an isometric view of the external fixation with the lower andupper rings oriented substantially parallel to each other;

FIG. 32 is an isometric view of the external fixation system of FIG. 26with the lower and upper rings oriented in a second angular position;

FIG. 33 is a schematic diagram of a six (6) degrees of freedom externalfixation system with RRUS kinematic chains, wherein “R” stands forrevolute join, “U” stands for universal joint, and “S” stands forspherical joint;

FIG. 34 is a vector model diagram of a six strut external fixationsystem, showing centers O and C; and

FIG. 35 is a vectorial model of a circle in Cartesian coordinate system.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, there is shown the external fixation systemof the present invention generally denoted as 10. The external fixationsystem 10 may be utilized with any long bone, in particular, the tibiaand the femur.

As shown in FIGS. 1 and 2, the external fixation system 10 includes afirst ring 14 and a second ring 16. In some embodiments, both rings 14,16 are identical. Each ring 14 includes a worm gear 15 formed around itsouter circumference. Two grooves 17 are formed in the upper and lowersurfaces of ring 14 around its circumference adjacent the worm gear 15.As shown in FIG. 1, ring 14 (or 16) may include a multi-levelconfiguration with the upper and lower surfaces having alternate stepsincluding through holes 24. Such an external fixation ring (without thecircumferential worm gear) is described in U.S. patent application Ser.No. 12/157,612 filed Jun. 11, 2008, the entire disclosure of which isincorporated herein be reference. In certain embodiments, rings 14, 16are connected by three variable length struts 18. The three struts 18have first ends 28 mounted to the first ring 14 via a connector 25coupled to a sliding or shuttle unit 26, which is circumferentiallymoveable around ring 14 as described below. In several embodiments, thefirst ends 28 are connected to sliding units or connector 26 by aconnector 25 having a ball or spherical joint. As is typical, the ringsare connected to tibia 12 by a plurality of bone pins or wires (notshown). In some embodiments, the pins or wires are connected to eachring 14, 16 by connection elements, which are located in one or more ofa multiplicity of holes 24 around the circumference of the first andsecond rings 14 and 16. Although holes 24 are shown, any structure whichlocates the pins or wires with respect to the circumference of rings 14and 16 can be utilized. Lower ends 34 of struts 18 are connected tolower ring 16 by standard universal-joints 35, which allow free rotationabout only two axes rather than the three axes of the spherical joint atthe first strut end 28.

Ring 14 may be coupled to a first bone element via pins or wires and,similarly, ring 16 is coupled to a second bone element by similar pinsor wires. Shuttle unit 26 is slidable about ring 14 in a track and ispreferably driven by a servo motor. A second connector 29 between strut18 and second lower ring 16 has a standard universal joint 35, whichallows the strut to rotate freely about first and second axes A and B(see FIG. 3A). First axis A is oriented perpendicular with respect tosecond axis B. The universal joint 35 may also be powered by servomotors independently connected to a gear rotatable about a pivot pinaxially oriented along axis A of universal joint 35. Thus, each of thethree sliding shuttle units 26 may be independently controlled and thethree connectors 29 at the second ring 16 may be independentlycontrolled so that the ring 14, and therefore the bone element attachedto ring 14, can be positioned in proper alignment with ring 16 and thebone element attached to ring 16. Rings 14 and 16 can be repositionedafter their initial alignment as desired by the surgeon. In addition,the movement can be programmed into a computer means, which canautomatically increment movement, for example, on a daily basis. Strut18 is of variable length but can be locked at a desired length after thesurgeon initially sets the starting location of the system.

Referring to FIGS. 3A and 3B, there is shown universal joint 35 couplinga second end 34 of rod 18 to the second ring 16. Preferably, end 34 isbifurcated and has a bore for receiving a pivot pin. A connector 36 isnon-rotatably connected to second ring 16 through one of themultiplicity of through holes 24 therein. Connector 36 includes a pairof bushings 38 through which a pivot pin 40 is received for rotationabout axis A. Pivot pin 40 is driven by a gear portion or worm gear 42which, in one embodiment, it is integral therewith. In the preferredembodiment, gear portion 42 is driven by a servo motor having a gearsystem with a drive connector or worm 43 for engaging gear teeth 44 ofgear portion 42. Pin 40 has a bore therethrough for receiving a pin 45pivotably coupling end of shaft or rod 18 thereto for rotation aboutaxis B. The bore in pin 40 receives pin 45, which is mounted on a pairof bushings 50 on bifurcated end portion 34. Referring to FIG. 3B, ahousing 60 is shown in which the miter gear 43 is retractably mounted.Miter gear 43 is mounted on shafts 52 and 54, which in turn are receivedwithin spring-loaded mounting elements 56 and 58. Housing 60 has acylindrical outer wall 60 as best seen in FIG. 3A. Mounting elements 56and 58 are biased inwardly by springs 62 and 64, which forces miter gear43 into engagement with the teeth 44 of gear portion 42. When a usermoves mounting elements 56 and 58 actually outwardly along shafts 52 and54, respectively, against the forces of springs 62 and 64, miter gear 43can move downwardly in FIGS. 3A and 3B out of engagement with teeth 44.Springs 62 and 64 then lock the mounting elements 56 and 58 in thisposition so that shaft 18 may be freely pivoted about axis A of FIG. 3A.

Referring to FIG. 3C, there is shown a cross-sectional elevation view ofthe connector of FIGS. 3A and 3B. This figure is useful for showing thedisengagement of worm 43 from teeth 44 of worm gear 42. Mountingelements 56 and 58 are slidably mounted on circular end plates 72 and74, respectively. End plates 72 and 74 are fixably coupled to drivesockets 76, 78, which in turn are integrally connected with ends 52 and54 of worm 43. Thus, when a drive tool is attached to either driveelement 76 or 78 and rotated, worm 43 will rotate and will engage withteeth 44, rotating worm gear 42 about axis A. This can also beaccomplished via a servo motor.

In order to disengage worm 43 from teeth 44, mounting elements 56 and 58are moved outwardly, thereby compressing springs 62 and 64 untilchamfers 80 and 82 engage ends 84 and 86 of the housing 60. At thispoint, worm 43 can move downwardly in FIG. 3C and out of engagement withteeth 44 of worm gear 42. In order to ensure that worm 43 maintains itscentral location within housing 60, four pins 88 are mounted on housing60, as shown in FIG. 3A, and engage in the circumferential grooves 90and 92 in worm 43. Grooves 90 and 92 have sufficient depth to allow worm43 to be moved into and out of engagement with teeth 44 of worm gear 42without disengaging from pins 88.

The connection system shown in FIGS. 3A to 3C allows the connector 29(FIG. 2) and, consequently shaft 18, to rotate about axis C of element36 and axis B of universal joint 35 freely. Free rotation about axis Ais only possible when worm 43 is disengaged from teeth 44 as describedabove. Otherwise, rotation is controlled by driving either input 76 or78, thereby rotating worm gear 43 while in engagement with teeth 44.

Referring to FIGS. 4A to 4J, there is shown the coupling system forconnecting shaft 18 with first ring 14.

Referring to FIG. 4B, there is shown shaft 18 connected by a universaljoint 100 which allows rotation about three axes D, E, and F. Connector26 is coupled to shaft 18 and includes a worm 106, which engages teeth15 of ring 14. This engagement is seen in FIGS. 4C through 4E in whichconnector 26 is shown being connected to ring 14 and locked thereon.

Referring to FIGS. 4C to 4J, there is shown ring 14 having a pair ofgrooves 17, which extend around the circumference of the ring adjacentteeth 15. Connector 26 includes a housing or body 108, which rotatablyreceives worm 106 and which has a first pair of pins 110 mounted on anarm 112 of housing 108. Each of pins 110 includes a head 114 adapted tobe inserted in groove 17 of ring 14. Connector 26 also includes amovable element 116, which may slide toward and away from housing 108 bythe action of lever 118, which includes a cam element 120. Lever 118 isrotatable about a pivot pin 122, which is mounted on movable element116. Movable element 116 also includes a second pair of pins 124 eachhaving a head 126, which, like head 114 of pin 110, can engage groove 17of ring 14. Pins 124 are fixed on moveable element 116.

As can be seen in the figures, lever 118 can be rotated into a positionin which a spring element (not shown) moves movable element 116 awayfrom engagement with body 108 of connector 26 and consequently movespins 124 upwardly in the figures, which allows connector 26 to be movedlaterally into engagement with the ring 14. As shown in FIG. 4D, oncethe connector 26 is properly positioned with respect to groove 17, lever118 can then be rotated as shown in FIG. 4E, such that the cam 120,which rotates about pivot pin 122, which is fixed with respect to body108, forces movable element 116 downwardly such that pins 124 areengaged in the upper groove 17 and pins 110 are engaged in the lowergroove 17. The heads 114 and 126 of pins 110 and 124 are sized withrespect to groove 17 such that the connector 26 may slide around thecircumference of ring 14. Pins 110 and 124 are spaced such that arecapable of engaging the grooves 17 of multiple diameter rings. As shownin FIGS. 4G and 4I, worm 106 of connector 26 can be moved into and outof engagement with the teeth 15 of ring 14 while the connector 26 ismounted on groove 17 of ring 14. This can be accomplished as shown inFIGS. 4H and 4J in which elements 130 and 132 can be moved with respectto portions 134 and 136 of the drive shaft for worm 106. Elements 130and 132, in the preferred embodiment are identical to elements 56 and 58described with respect to FIGS. 3A and 3C. The other elements of thesystems, including springs 62 and 64 are also identical. Thus, FIGS. 4Gand 4H show worm 106 in a position which is out of engagement with teeth15 of ring 14 with FIGS. 4I and 4J showing worm 106 moved into aposition where the worm 106 engages the teeth 15 of ring 14.

Referring to FIG. 4F, there is shown an alternate drive system for theshuttle or sliding unit 26 of the present invention. As with theconnector shown in FIGS. 3A to 3C, unit 26 is driven around thecircumference of ring 14 by turning drive elements 140 and 142. This maybe done either with a hand tool or a power tool. As can be seen fromFIG. 4G, the drive is preferably either a male or female square driveelement. The alternate system shown in FIG. 4F includes worm gear 106mounted in a lower base portion 127 and connected to an upper portion(not shown) by pins 129 a bevel gear 138 fixably mounted on either shaftportion 134 or 136 of worm 106. A drive shaft 140 extending generallyperpendicular to the axis of shaft portions 134 and 136, which shaft 140includes a drive gear 142 may also be used to drive worm 106.

Referring to FIG. 4A, there is shown an enlarged view of an alternatering 14, including a shuttle or sliding unit 26 mounted on ring 14.Shuttle unit 26 is coupled to a first end 28 of each strut 18. Shuttleunit 26 includes a drive system for moving the shuttle about thecircumference of ring 14. The drive system includes a server or steppermotor which is controlled by software on a computer, which determinesadjustments to the location of shuttle 26 about the circumference ofring 14 during use of the external fixation system. These adjustmentsare performed on a daily basis. The computer system softwareincorporates the input from potentiometers and/or optical encodersand/or other position sensors at the various joints in the externalfixation system to determine both the initial position of the system aswell as to confirm that the daily adjustments are being made properly.

The ends 28 of strut 18 are connected to shuttle 26 via a standarduniversal joint-type connector. As shown in FIGS. 4A, 4B, a pivot pin 30mounted within a pair of bushings 32 on axis or rotation of theuniversal joint with an additional axis of rotation coaxial with thelongitudinal axis of each strutting team and a third axis similar toaxis 30 thus forming a typical universal joint.

In some embodiments, sliding unit 26 in the gear portion 42 are drivenby a worm gear, which in turn is driven by a stepper or servo motorhaving an output shaft with a bevel gear, which may be a miter gear. Assuch there are three independent struts having movable first and secondends and 34 connected to the first and second rings, respectively. Eachof the three struts 18 may be moved around the circumference of thefirst ring 14 by the stepper/slash servo motors driving sliding unit 26.In addition, the second end 34 of rod 18, although circumferentiallyfixed in a single hole 24 of ring 16, can be rotated in planesperpendicular to the plane of ring 16 by its dedicated stepper or servomotor. The combination of these movements is capable of orienting ring16 and ring 14 in an infinite number of angular positions with respectto one another. This change in orientation can be accomplished withfixed length struts 18.

The external fixation system of the present invention is normallysupplied as a kit with a plurality of rings of different diameters, someof which are either fully circular rings or partial rings allowing theirplacement over the limb to be treated in a medial-lateral direction. Inaddition, struts 18 of various fixed lengths can be provided in the kitto produce various axial distances between the centers of the first andsecond rings 14, 16, respectively. Each strut 18 supplied has first andsecond ends 28, 34 capable of being connected to the sliding unit 26 andsecond ring connector 29 as described above.

A controller will also be provided, including microprocessors programmedto implement the various inputs to these six steppers or servo motors ofthe system. With reference to FIGS. 5-7, the following is a mathematicaldescription of the upper (moving) ring 14 and the base points on thelower (relatively fixed) reference ring 16. We will describe the upperring as a circle on a plane. The three points P_(B1), P_(B1) and P_(B3)represent the fixed base points on the lower ring 16. The two rings 14,16 are connected by three struts 18. Each strut 18 is capable ofrotating about its fixed base point (P_(B1), P_(B2) or P_(B3)) andconnects to the upper ring 14 at their respective end points (P1, P2 andP3). The end points (P1, P2 and P3) can lie anywhere on the perimeter ofthe upper ring 14. The range of motion of the end point of a strut (P1,P2 or P3) can be thought of as the surface of a sphere whose center isthe strut's base point and whose radius (r) is the length of the strut.This is visually represented in FIG. 6. The base points are fixed anddescribed in FIG. 5. The length between the base points (PB1, PB2 andPB3) is d. The upper ring 14 has a radius R1.

Any Point in Space

P(X, Y, Z)X=r sin(θ)cos(φ)Y=r sin(θ)sin(φ)Z=r cos(θ)The Points at the End of the StrutsThese make up the three coplanar points that will connect the struts tothe upper ring. They will always be coplanar as they are all connectedto a common ring.Point 1 (P₁i):X ₁ =r sin(θ)cos(φ)Y ₁ =r sin(θ)sin(φ)Z ₁ =r cos(θ)Point 2 (P₂):X ₂ =r sin(θ)cos(φ)+d sin(15)Y ₂ =r sin(θ)sin(φ)+d cos(15)Z ₂ =r cos(θ)Point 3 (P₃s):X ₃ =r sin(θ)cos(φ)+d cos(15)Y ₃ =r sin(θ)sin(φ)+d sin(15)Z ₃ =r cos(θ)The Vectors from P1 to P2 and P1 to P3Using the three points at the end of the struts, we can find two vectorson the plane.P1P2=â=<(X2−X1),(Y2−Y1),(Z2−Z1)>P1P3=b<(X3−X1),(Y3−Y1),(Z3−Z1)>The Normal Vector of Plane 2 that the Points P1, P2 and P3 Sit onUsing the two vectors on the plane, we can find the normal vector.

$\begin{matrix}{n = {\hat{a} \times b}} \\{= {{\left( {\left( {{Y\; 2} - {Y\; 1}} \right)\left( {{Z\; 3} - {Z\; 1}} \right)} \right)i} +}} \\{{\left( \left( {{Z\; 2} - {Z\; 1}} \right) \right)\left( \left( {{X\; 3} - {X\; 1}} \right) \right)j} +} \\{{\left( {\left( {{X\; 2} - {X\; 1}} \right)\left( {{Y\; 3} - {Y\; 1}} \right)} \right)k} -} \\{{\left( {\left( {{Z\; 2} - {Z\; 1}} \right)\left( {{Y\; 3} - {Y\; 1}} \right)} \right)i} -} \\{{\left( {\left( {{X\; 2} - {X\; 1}} \right)\left( {{Z\; 3} - {Z\; 1}} \right)} \right)j} -} \\{\left( {\left( {{Y\; 2} - {Y\; 1}} \right)\left( {{X\; 3} - {X\; 1}} \right)} \right)k} \\{= {{\left( {{\left( {{Y\; 2} - {Y\; 1}} \right)\left( {{Z\; 3} - {Z\; 1}} \right)} - {\left( {{Z\; 2} - {Z\; 1}} \right)\left( {{Y\; 3} - {Y\; 1}} \right)}} \right)i} +}} \\{{\left( {{\left( {{Z\; 2} - {Z\; 1}} \right)\left( {{X\; 3} - {X\; 1}} \right)} - {\left( {{X\; 2} - {X\; 1}} \right)\left( {{Z\; 3} - {Z\; 1}} \right)}} \right)j} +} \\{\left( {{\left( {{X\; 2} - {X\; 1}} \right)\left( {{Y\; 3} - {Y\; 1}} \right)} - {\left( {{Y\; 2} - {Y\; 1}} \right)\left( {{X\; 3} - {X\; 1}} \right)}} \right)k}\end{matrix}$For simplicity's sake, we'll set vector “n” to: n=<a, b, c>The Equation of “Plane 2” that P1, P2 and P3 sit onThe following describes the upper ring's plane at any given time.General equation of a Plane: AX+BY+CZ+D=0To solve for the equation of the plane we must find A, B, C and D bysetting the determinant of the matrix below equal to zero

${\det{\begin{matrix}{x - {x\; 1}} & {y - {y\; 1}} & {z - {z\; 1}} \\{{x\; 2} - {x\; 1}} & {{y\; 2} - {y\; 1}} & {{z\; 2} - {z\; 1}} \\{{x\; 3} - {x\; 1}} & {{y\; 3} - {y\; 1}} & {{z\; 3} - {z\; 1}}\end{matrix}}} = {{{X\left\lbrack {{y\; 3\left( {{z\; 1} - {z\; 2}} \right)} + {y\; 1\left( {{z\; 2} - {z\; 3}} \right)} + {y\; 2\left( {{{- z}\; 1} + {z\; 3}} \right)}} \right\rbrack} + {Y\left\lbrack {{x\; 3\left( {{{- z}\; 1} + {z\; 2}} \right)} + {x\; 2\left( {{z\; 1} - {z\; 3}} \right)} + {x\; 1\left( {{{- z}\; 2} + {z\; 3}} \right)}} \right\rbrack} + {Z\left\lbrack {{{- x}\; 2*y\; 1} + {x\; 3*y\; 1} + {x\; 1*y\; 2} - {x\; 3*y\; 2} - {x\; 1*y\; 3} + {x\; 2*y\; 3}} \right\rbrack} + \left\lbrack {{x\; 3\;*y\; 2\;*z\; 1} + {x\; 2*y\; 3*z\; 1} - {x\; 3*y\; 1*z\; 2} + {x\; 1*y\; 3*z\; 2} + {x\; 2*y\; 1*z\; 3} - {x\; 1*y\; 2*z\; 3}} \right\rbrack} = 0}$Where the coefficient of X is A, the coefficient of Y is B, coefficientof Z is C and the rest is the constant D.

Equation of Sphere

To solve for the upper ring we must find the equation of a sphere. Thissphere will share the upper ring's center point and radius “R1”. Thesphere will also have P1, P2 and P3 on its surface. The plane we solvedfor above passes through the sphere's center and contains P1, P2 and P3.Therefore, the intersection of this sphere and the plane will describethe equation of the circle we are ultimately solving for to representthe upper ring.

Since P1, P2 and P3 are on the sphere's surface, the distance from thesepoints to the center of the sphere will be equal. Setting up thefollowing three equations will allow us to solve for the center point(Xc, Yc, Zc).

Given R1, P1, P2, P3 and the General Equation of a Sphere:(X−X _(center))^2+(Y−Y _(center))^2+(Z−Z _(center))^2=R1^2We can solve for (Xc, Yc, Zc):(X1−Xc)^2+(Y1−Yc)^2+(Z1−Zc)^2=R1^2And(Xc−X1)^2+(Yc−Y1)^2+(Zc−Z1)^2=(Xc−X2)^2+(Yc−Y2)^2+(Zc−Z2)^2And(Xc−X1)^2+(Yc−Y1)^2+(Zc−Z1)^2=(Xc−X3)^2+(Yc−Y3)^2+(Zc−Z3)^2Solve for (Xc, Yc, Zc)

The Cartesian Equation of the Circle

Given the center (Xc, Yc, Zc) (from the sphere above), the normal vectorn=<a, b, c> (from the plane) and the three points P1, P2 and P3 (on thecircle); the Cartesian representation of the circle is:(X−Xc)^2+(Y−Y _(c))^2+(Z−Zc)^2=R1^2AndX[y3(z1−z2)+y1(z2−z3)+y2(−z1+z3)]+Y[x3(−z1+z2)+x2(z1−z3)+x1(−z2+z3)]+Z[−x2*y1+x3*y1+x1*y2−x3y2−x1*y3+x2*y3]+[x3*y2*z1+x2*y3*z1−x3*y1*z2+x1*y3*z2+x2*y1*z3−x1*y2*z3]=0

Parametric Equation of Circle

With the center being (Xc, Yc, Zc) (from the sphere above) and thenormal vector being n=<a, b, c> (from the plane), the Parametricrepresentation of the ring is:X(t)=Xc+(a*c*R*cos(t)−b*R*sin(t)/(a^2+b^2)^(½)Y(t)=Yc+(b*c*R*cos(t)+a*R*sin(t)/(a^2+b^2)^(½)Z(t)=Zc−R*cos(t)*(a^2+b^2)^(½)Where: 0≦t≦2 n

EXAMPLE

Thus, to align a first bone element with respect to a second boneelement, one utilizes the above mathematical model to design software.The software will first consider the initial position of the rings withrespect to the bone elements. The final position of the frame with thealigned bones will be determined by the software, taking into accountthe size and position of the rings and struts. The software willcalculate the shortest trajectory from the initial to final position ofthe moving ring, generating the intermediate positions of the movingelements on the ring, and the angular rotations of the struts at thefixed ends, using a form of ring kinematics to get the necessary values.The ring kinematics will be derived by applying the mathematical formulaabove to determine the iterations required to get from the initialposition to the final position. These iterations will be generated witha constraint on the maximum possible correction per day as defined bythe surgeon in terms of the maximum distraction rate.

Referring to FIGS. 8-13, there is shown an alternate embodiment of thepresent invention. Referring to FIG. 8, there is shown a manipulatorhaving a pair of rings identical to the rings 14 and 16 of the preferredembodiment. Ring 14 includes three sliding units 26 which are identicalto those shown in FIGS. 4B-4J. Three identical struts 200 are utilizedin the alternate embodiment which struts have an adjustable length. Inthis embodiment, the struts are connected to the sliding units 26 via ayoke 202, which is shown in FIGS. 11 and 12 and which allows rotationabout a single axis G. The other end of each strut 200 is connected tothe ring 16 via an expandable coupling element 204, which is inserted inone of the holes 24 in ring 16. As will be discussed below, yoke 202 isattached to sliding unit 26 by a ball and socket joint. As shown inFIGS. 9 and 10, expandable coupling 204 is connected to one element ofthe strut 200, which may be a threaded rod 206 to allow adjusting of thelength of the strut 200. The connection between the coupling element 204and the threaded rod 206 is by a ball joint 208 which allows rotationabout three axes. Since threaded rod 206 is received within a threadedbore (not shown) in the upper portion of strut 200, rotation of rod 206about the ball joint 208 along the longitudinal axis of the rod 206increases or decreases the length of the strut 200.

Sliding unit 26 operates as described above in connection with theembodiment depicted in FIG. 2 to move the upper ends of the struts 18around the circumference of ring 14. When not being driven, sliding unit26 is fixed firmly in position on ring 14. Attaching the upper end ofyoke 202 to the sliding unit 26 is via a similar ball and socket jointas ball and socket joint 208 which will allow the strut and sliding unitto articulate relative to each other about the three rotational axes ofa spherical joint, allowing for three degrees of freedom in thekinematic chain. Alternately, strut 18 could attach to a rotationallyfixed, separate piece which would form the socket. As shown in FIG. 10,in order to keep the spherical joint in tact while unloaded, it isdesired to include a separate cap 212, which threads onto the end of rod206 to ensure that the ball does not slip out of the socket.

As shown in FIG. 13 strut 200 is, as discussed above, formed out of twopieces, one being threaded rod 206 and the upper section including apart 203 with a threaded bore receiving rod 206.

Referring to FIG. 14, an alternate embodiment of the presently disclosedexternal fixation system is generally designated as 300. Externalfixation system 300 includes a first platform or ring 314, a secondplatform or ring 316, and a plurality of non-prismatic kinematic chainsor struts 390 each connecting the first platform 314 to the secondplatform 316. For the purposes of the present disclosure, the term“prismatic” means an element or joint with one translational degree offreedom. Usually, prismatic kinematic chains or struts are extendableand can therefore vary their lengths along their longitudinal axisduring actuation. On the contrary, the term “non-prismatic” refers to ajoint or element incapable of changing its length along its longitudinalaxis upon actuation. Thus, the term “non-prismatic kinematic chain,” asused herein, means a kinematic chain that does not varies its lengthalong its longitudinal axis during actuation.

In the embodiment depicted in FIG. 14, external fixation system 300includes three non-prismatic kinematic chains 390 but external fixationsystem 300 may have more non-prismatic kinematic chains 390. Forexample, certain embodiments of external fixation system 300 may havefour or even six non-prismatic kinematic chains 390. Regardless of thespecific number of non-prismatic kinematic chains 390, each kinematicchain 390 connects the first platform 314 to the second platform 316 andhas a first end portion 352 and a second end portion 354. First endportions 352 of each kinematic chain 390 are connected to first platform314, whereas second end portions 354 of each kinematic chain 390 areconnected to second platform 316. As a consequence, kinematic chains 390maintain first and second platforms 314, 316 spaced apart from eachother. However, the distance and orientation between first and secondplatforms 314, 316 may vary as one or more kinematic chains 390 moverelative to one another.

First and second platforms 316, 314 are substantially similar to firstand second rings 14, 16, as described above, and to each other.Nevertheless, first and second platforms 314, 316 may be made ofdifferent materials. For example, in certain embodiments, first platform314 is wholly or partly made of aluminum, while second platform 316 iswholly or partly made of a radiolucent carbon fiber or a reinforcedpolymer such as polyetheretherketone (PEEK).

As seen in FIG. 15, second platform 316 has a chamfer 319 extendingalong its circumference in addition to holes 324, groove 317, and wormgear 315. As with second ring 16, second platform 316 has grooves 317 onopposite sides. Grooves 317 extend along the circumference or perimeterof second platform 316. Chamber 319 is located radially outwardly withrespect to groove 317 and facilitates connection of kinematic chains 390to second platform 316 as discussed in detail below. In someembodiments, second platform 316 has a planar configuration and definesa plane. In these embodiments, an axis J extends orthogonally relativeto the plane defined by second platform 316. First platform 314 issubstantially similar to second platform 316 and also includes a chamfer319 extending along its circumference (see FIG. 14).

With reference to FIGS. 16-17, each kinematic chain 390 includes a shaft318 positioned between first and second end portions 352, 354, aspherical joint 308 located at first end portion 352, a universal joint335, and a sliding unit 326 positioned at second end portion 354. Shaft318 has a first end 328 and a second end 334 and connects universaljoint 335 to spherical joint 308. In particular, universal joint 335 isconnected to first end 328 of shaft 318, while spherical joint 308 iscoupled to second end 334 of shaft 318. As shown in FIGS. 18 and 19,shaft 318 is hollow and has a bore 341 extending between first andsecond ends 328, 334. Shaft 318 may be wholly or partially made of aradiolucent material. In certain embodiments, shaft 318 is made of atitanium alloy (Ti6Al4V). Second end 334 of shaft 318 includes a socket351, which is part of spherical joint 308.

With reference to FIGS. 18-19, spherical joint 308 includes a socket 351formed within second end 334 of shaft 318 and a ball 353 dimensioned tobe received in socket 351. In operation, spherical joint 308 providesthree degrees of rotational freedom and facilitates movement ofkinematic chain 390 with respect to first platform 314. A cap 355 ispositioned around second end 334 of shaft 318 and engages ball 353,thereby maintaining a portion of ball 353 within socket 351 withoutinhibiting the movement of ball 351. A connecting member 357 extendsfrom ball 351 and is dimensioned to be received within a hole 324 ofeither first platform 314 or second platform 316. Connecting member 357includes a threaded end portion 359 (FIG. 16) adapted to mate with anut. 361 (see FIG. 14) and a base end portion 363 having a flat surface365. When spherical joint 308 is connected to first platform 314, flatsurface 365 of base end portion 363 engages one side of first platform314, while nut 361 engages another side of first platform 314. Toconnect spherical joint 308 to first platform 314, connecting member 357is inserted through one of the holes 324 of first platform 314 untilflat surface 365 of base end portion 363 engages one side of firstplatform 314. Then, nut 361 is threaded onto threaded end portion 359.After threading nut 361 onto threaded end portion 359 of connectingmember 357, nut 361 fastens spherical joint 308 to first platform 314,as seen in FIG. 14.

Referring again to FIGS. 16-19, sliding unit 326 includes a firstrevolute joint 395 and universal joint 335, which itself includes secondand third revolute joints 337, 339. Universal joint 335 is substantiallysimilar to universal joint 35 (FIG. 3A). Second and third revolutejoints 337, 339 provide universal joint 335 with two degrees ofrotational freedom. Second revolute joint 337 is configured to rotateabout axis H, while third revolute joint 339 is adapted to rotate aboutaxis I. Axis H is oriented substantially orthogonal to axis I. A yoke orhousing 350 connects universal joint 335 to first end 328 of shaft 318and holds a pivot pin 345 axially aligned along axis H.

Referring to FIGS. 20 and 21, yoke 350 includes two legs 371 orientedsubstantially parallel to each other. Each leg 371 includes a bore 373longitudinally aligned with axis H and dimensioned to receive pivot pin345. Pivot pin 345 extends through bores 373 of yoke 350 and pivotallycouples a gear portion or worm gear 342 of universal joint 335 to yoke350. Gear portion 342 is substantially similar to gear portion 42 (FIG.3A) and includes a driving section 375 and a connecting section 377extending from driving section 375. Connecting section 377 is sized tobe received between legs 371 of yoke 350 and has a bore (not shown)substantially aligned with axis H and dimensioned to receive pivot pin345. When assembled, pivot pin 345 extends through bores 373 of yoke 350and the bore of driving section 375 aligned with axis H, therebypivotally coupling gear portion 342 to yoke 350. As a result of thismechanical arrangement, second revolute joint 337 can rotate about pivotpin 345, which is positioned substantially parallel to axis H.

With continued reference to FIGS. 20 and 21, gear portion 342 ofuniversal joint 335 aids in the rotation of third revolute joint 339. Tothis effect, connecting section 377 of gear portion 342 has a bore 379axially aligned with axis I (see FIG. 16). As discussed in detail below,bore 379 is dimensioned to receive pivot pin 340. Aside from bore 379,gear portion 342 includes gear teeth 344 formed along a perimeter ofdriving section 375. In the depicted embodiment, driving section 375 hasa semi-circular shape but driving section 375 may have any othersuitable shape or configuration. Driving section 375 of gear portion 342further includes markings indicating the angulation of shaft 318 withrespect to sliding unit 326 (FIG. 14).

Referring again to FIGS. 16-19, gear teeth 344 of gear portion 342 areconfigured to engage the teeth of drive connector or worm 343. Driveconnector 343 is substantially similar to worm 43. Worm 343 and gearportion 342 collectively form a worm gear drive system and are part ofthird revolute joint 339. As discussed above, third revolute joint 339can rotate pivot pin 340, which is axially aligned with axis I. Pivotpin 340 is dimensioned to be received in bore 379 (FIG. 21) of gearportion 342 and pivotally connects gear portion 342 to a clamp body 383of sliding unit 326.

Clamp body 383 of sliding unit 326 holds parts of third revolute joint339, i.e., gear portion 342 and worm 343. Worm 343 is configured toengage gear portion 342. As a result, gear portion 342 pivots about axisI upon rotation of worm 343 about axis K when gear portion 342 and worm343 are engaged to each other. As discussed with regard to driveconnector 43, drive connector 343 can be driven by any suitablemechanical or electro-mechanical tool or means. For example, worm 343may be driven by a “smart tool” as described in U.S. Pat. No. 6,017,354,a dedicated stepper/servo motor or a hand tool. Since third revolutejoint 339 can be pivoted about pivot pin 340 through the actuation ofworm 343, third revolute joint 339 is deemed an actuated joint. For thepurposes of the present disclosure, an “actuated joint” means any jointcapable of being driven or actuated by a mechanical orelectro-mechanical tool. In the embodiment illustrated in FIG. 16, therotation of worm 343 about axis K causes the pivotal movement of thirdrevolute joint 339 about pivot pin 340. Pivot pin 340 pivotally couplesgear portion 343 to clamp body 383.

With reference to FIGS. 22 and 23, clamp body 383 of sliding unit 326has a first end 381 and a second end 385 and includes a pair of legs 387extending toward first end 381. Legs 387 are oriented substantiallyparallel to each other and define a space 391 between them dimensionedto receive gear portion 342 (FIG. 16). Each leg 387 includes a bore 389dimensioned to receive pivot pin 340. Bores 389 are both axially alignedwith axis I (FIG. 16). Pivot pin 340 (FIG. 16) extends through bores 389of clamp body 383 and bore 379 of gear portion 342 (FIG. 21), connectingclamp body 383 to gear portion 342. Space 391 leads to an opening 393dimensioned for receiving worm 343. Worm 343 is retractably mountedwithin opening 393. Accordingly, worm 343 is configured to move betweenan engaged position relative to gear portion 342 and a disengagedposition with respect to gear portion 342. In the engaged position, worm343 mates with gear portion 342 and, consequently, worm 343 can drivegear portion 342 when rotated about axis K. In the disengaged position,worm 343 does not mate with gear portion 342 and, therefore, cannotdrive gear portion 342.

The structure and operation enabling movement of worm 343 between theengaged and disengaged positions are identical to the structure andoperation of worm 43 (see FIGS. 3B and 3C). For example, worm 343 ismounted on shafts (not shown), which are in turn received withinspring-loaded mounting elements 356 and 358. Mounting elements 356 and358 are identical to mounting elements 56 and 58. Moreover, mountingelements 356 and 358 are biased inwardly by springs (not shown), whichforces worm 343 into engagement with teeth 344 of gear portion 342. Whena user moves mounting elements 356 and 358 actually outwardly along theshafts against the force of the springs, the worm 343 can move towardgear portion 342 and into engagement with teeth 344. The springs thenlock mounting elements 356, 358 in the engaged position so that shaft318 may be freely pivoted about axis I (FIG. 16). In addition, worm 343includes drive elements 376, 378 integrally connected at its ends. Driveelements 376, 378 are identical to drive elements 76, 78. In operation,drive elements 376, 378 facilitate attachment of a driving tool to worm343. Thus, when a drive tool is attached to either drive element 376 or378 and rotated, worm 343 rotates about axis K and engages teeth 344,thereby rotating gear portion 342 about axis I (FIG. 16).

Referring again to FIGS. 16-19, each kinetic chain 390 includes a thirdrevolute joint 395 capable of rotating around a perimeter orcircumference of second platform 316. Third revolute joint 395 is partof sliding unit 326 and includes a worm 406 that is identical to worm106 (FIG. 4H).

With reference to FIGS. 22 and 23, worm 406 is retractably mounted in anopening 493 formed adjacent to second end 385 of sliding unit 326. Worm406 can move between engaged and disengaged positions relative to wormgear 315 of second platform 316 (FIG. 14). In the engaged position, worm406 mates with worm gear 315 of second platform 316, whereas, in thedisengaged position, worm 406 does not mate with worm gear 315 of secondplatform 315. Worm 406 includes spring-loaded mounting elements 456 and458 for facilitating engagement and disengagement with worm gear 315 ofsecond platform 316. The structure and operation of mounting elements456 and 458 are identical to the structure and operation of mountingelements 356 and 358. To move worm 406 between the engaged anddisengaged positions, the user should follow procedure discussed abovewith regard to worm 343. In the case of worm 406, however, the usermoves worm 406 toward or away from worm gear 315 after moving mountingelements 458 and 458 outwardly to disengage or engage worm 406 with wormgear 315. In addition, worm 406 includes drive elements 476, 478integrally connected at its ends. Drive elements 476, 478 are identicalto drive elements 376, 378 and facilitate attachment of a driving toolto worm 406.

When worm 406 is located in the engaged position relative to worm gear406, worm 406 can revolve along the perimeter or circumference of secondplatform 316 upon rotation of worm 406 about axis L. Worm 406 can bedriven (that is, rotated about axis L) with any of suitable mechanicalor electro-mechanical tool or means such as a “smart tool” a dedicatedstepper/servo motor or a hand tool. Given that first revolute joint 395can be actuated through the rotation of worm 406 about axis L, firstrevolute 395 is deemed an actuated joint. As discussed above, thirdrevolute joint 339 is also considered an actuated joint. Sphericaljoints 308 are not considered actuated joints because these joints arenot driven or actuated. Therefore, each kinematic chain 390 of theembodiment shown in FIG. 14 includes at least two actuated joints. Eachkinematic chain 390, however, may include more actuated joints.

With continued reference to FIGS. 22 and 23, sliding unit 326 includes abase 412, a movable arm 416 adapted to move toward and away from base412, a first pair of pins 410 mounted on base 412, and a second pair ofpins 424 mounted on movable arm 416. An open area or space is definedbetween movable arm 416 and base 416 and is dimensioned to receiveeither first or second platform 314, 316. This space allows a user tosnap sliding unit 326 onto either first platform 314 or second platform316. First and second pair of pins 410, 424 also help in the connectionbetween sliding unit 426 and either first platform 314 or secondplatform 316.

The structure and operation of pins 410 are identical to the structureand operation of pins 110 (see FIGS. 4C-4E), and the structure andoperation of pins 424 are identical to the structure and operation ofpins 124 (see FIGS. 4C-CE). Briefly, each pin 410 has a head 414 adaptedto engage groove 317 of either first platform 314 or second platform316. Likewise, each pin 424 has a head 426 configured to engage groove317 of either first platform 314 or second platform 316. In operation,heads 414 of pins 410 slide along one groove 317, while heads 424 ofpins 424 slide along another groove 317 of the same platform (314 or316) as sliding unit 326 revolves around the circumference of saidplatform (314 or 316).

Sliding unit 326 additionally includes a first sheet 450 mounted on base412 for maintain the position of the first pair of pins 410 and a secondsheet 452 mounted on movable arm 416 for maintain the position of thesecond pair of pins 424.

As discussed above, movable arm 416 can move toward and away from base412. In some embodiments, a bolt 418 or any other suitable apparatuscontrols the movement of movable arm 416 relative to base 412 and helpssecure sliding unit 426 to second platform 416. Clamp body 383 includesa threaded hole 460 positioned and dimensioned to receive and engagebolt 418. Bolt 418 can secure movable arm 416 to clamp body 383 whensecurely received within threaded hole 460. A user can move movable arm416 toward or away from base 412 by screwing or unscrewing bolt 418 fromthreaded hole 460.

Referring to FIG. 24, a measuring device is generally designated as 500.Measuring device 500 is effectively a measuring tape having a first end502 and second end 504. First end 502 includes a magnetic component 506adapted to be connected to drive elements 476 or 478 of worm 406 throughmagnetism. Magnetic component 506 includes two or more links 508 or anyother suitable articulation mechanism. Links 508 allow magneticcomponent 506 to articulate relative to second end 504. In operation, auser utilizes measuring device 500 to measure the distance between anend of a worm 406 located in one kinematic chain 390 and a sphericaljoint 308 located in another kinematic chain 390. In the case of anexternal fixation system with three kinematic chains 390, the usershould measure this distance three times. Each time the user shouldmeasure such distance between a different pair of kinematic chains 390.Each measurement in effect represents the length of a “phantom strut” ofa hexapod, making calculations for the software much simpler.

FIG. 25 shows another measuring apparatus 600 for measuring the distancebetween the osteotomy site and second platform 316. Osteotomy site referto the location where the surgeon cuts the bone in two segments beforefixing the resulting two bone segments with an external fixation system.Measuring apparatus 600 can be coupled to magnetic component 506 ofmeasuring device 500 through magnetism and includes a planar portion 602configured to be aligned with the osteotomy site while measuring, acoupling portion 604, and a handle 606 located between planar portion602 and coupling portion 604. In use, magnetic component 506 ofmeasuring device 500 is initially secured to coupling portion 604 ofmeasuring apparatus 600. Then, the user aligns planar portion 602 withthe osteotomy site and subsequently measures the distance between theosteotomy site and second platform 316.

In use, a physician may employ external fixation system 300 as well asthe alternate embodiments to perform an osteotomy. Osteotomy may beperformed at any long bone such as the tibia and the femur. In anexemplary method, the physician attaches the first platform to a firstbone segment with any suitable apparatus such as wires or pins. Then,the physician attaches the second platform to a second bone segment withwires or pins. After securing the first and second platforms todifferent bone segments, the physician should determine the properrelative position of the first bone segment with respect to the secondbone segment (i.e., a predetermined position). Using the softwaredescribed above, the physician then uses a mathematical correlation ofthe relative position of the first platform with respect to the secondplatform to determine the new locations for the actuated joints requiredto reposition the first bone segment to the predetermined position withrespect to the second bone segment. Next, the physician actuates theactuated joints to move said actuated joints to the new determinedlocations. Other methods of utilizing the disclosed external fixationsystem are envisioned. Irrespective of the methods employed, thepresently disclosed external fixation system provide at least sixdegrees of freedom.

These components come together to allow the full assembly six degrees offreedom: three translational (x,y,z), and three rotational (pitch, roll,yaw). It is worth noting that the assembly does this with three strutsinstead of the normal six associated with a Gough/Stewart platform. Themathematics of this system is described in Alizade et al. Mech. Mach.Theory Vol. 29, No. 1, pp. 115-124, 1994 which is incorporated herein byreference in its entirety.

Many authors have proposed six degrees of freedom robots with only threelegs that will have two actuators per leg (hence they are not fullyparallel). This allows one to decrease the risk of interference betweenthe legs (thereby increasing the workspace size), but has the drawbackof reducing the stiffness while increasing the positioning errors.

Each of the three kinematic chains connecting the bottom ring to the topdemonstrates six degrees of freedom from its joints in the configurationshown. The first comes from the rotation of the sliding unit about thering 14. The second comes from the rotation of the strut about thesliding unit 26 via yoke joint 202. The third comes from the extensionof the strut via its prismatic joint. The final three come from thethree degrees of rotation allowed for by the spherical or ball joint. Tobe defined as a parallel robot, the design must have the same number ofactuators as it has degrees of freedom. As there are six degrees offreedom, there are six actuators: one prismatic (within the strut) andone rotational (between the sliding unit 26 and the gear) for each ofthe three legs. Each actuator provides the upper ring with one degree offreedom. Alizade et al. (id.) have explored the range of motion in asetup such as this already, demonstrating the size of the assembly'sworkspace and analyzing both forward and rear displacement. They alsodeclared that this assembly has a distinct advantage over theSteward/Gough platform in its ability to produce pure rotation.

The six degrees of freedom provided by these designs allow it the uniqueproperty of having a “virtual hinge.” When repairing a deformed bone, itis essential that re-alignment takes place centered on the Center ofRotation of Angulation (CORA)—the point at which the proximal mechanicalaxis and distal mechanical axis intersect. In older systems (e.g.Ilizarov), it was essential to build a physical hinge into the assemblythat aligned perfectly with the CORA. If a physician noticed halfwaythrough the patient's treatment that the alignment of this hinge wasoff, it became necessary to physically repair the system and repositionthe hinge. The virtual hinge afforded by six degrees of freedom greatlysimplifies this process. No actual hinge must be installed initially;the two rings are able to generate rotation about any single line,forming the “virtual hinge” there. If a physician notices that theinitially chosen line was inaccurate, all that must be done to fix theprescription is to simply correct the line acting as the virtual hinge.This can quickly and easily be done using software.

FIGS. 26 and 27 illustrate an embodiment of an external fixation systemwith six struts for manipulating the relative position of bone fragmentsto one another. The system is capable of moving in six axes of rotation.For movement, the system may incorporate either the use of the Strykerpatented “smart tool” (U.S. Pat. No. 6,017,354) or six dedicated servoor stepper motors. The smart tool or servo/stepper motors are to becontrolled by software. The software provides an interface for use bythe surgeon or user to determine daily adjustments. The system alsoincorporates potentiometers or optical encoders (at the moving pointsalong the frame) to not only determine the initial position (forsoftware input/setup) but also to insure that the daily adjustments arebeing made properly.

Each of the six struts 718 has a proximal and distal end. At theproximal end, the strut is connected to a ring with a sliding unit 726.At its distal end, it is bolted to the opposing ring. The six slidingunits 726 move about the perimeter of the rings 714, 716 to adjust theeffective distance between the rings. Sliding in the direction thatincreases the angle θ (FIG. 27) between the frame and the strutincreases the distance between the rings. Sliding in the oppositedirection decreases the distance.

When the external fixation system 700 is being set up in surgery, thestruts' length can be changed to attain the optimal starting position(Optimal Position, see FIG. 29). This is important because there arecertain positions where the struts 718 can interfere with one another,thus limiting the amount of movement possible before strut interference.Once the setup is complete and correction may begin, the struts arelocked and the length is fixed. The shuttle units 726 then moved aboutthe rings 714, 716 to adjust the ring's relative position in six axes ofadjustment. When the rings 714, 716 are being moved by the system, thelength of the strut 718 is constant and does not adjust.

When the struts are all equal length and each sliding unit's distal endis closest to the proximal end of its neighboring strut (see FIG. 28),the two rings 714, 716 are positioned parallel and vertically inline(see FIG. 29.)

Whenever the initial configuration of the struts 718 is such that thedistal end of each strut is closest to the proximal end of itsneighboring strut, see FIG. 28, it allows for the most optimal startingpoint and will be referred to as the “optimal position.” From thisposition the frame is ideally situated to be able to make correctionsbefore strut interference occurs.

Strut interference occurs when one strut's position prohibits anotherstrut from moving past. This effectively limits the range of motion. Bysetting up the system 700 in the optimal position, one limits theeffects interference has on the range of motion, thus maximizing thepossible adjustments from the starting point.

In many cases the rings 714, 716 will not be parallel and inlineimmediately following surgery. If the struts were all the same lengthand the rings 714, 716 were not parallel, one could not achieve theoptimal position of the struts. It is for this reason that the strutsmust be adjustable in length. Ideally, the system 700 will be positionedin the optimal position regardless of the relative position of one ring714, 716 to another. As can be seen in FIG. 30, the strut lengths aredifferent and the rings 714, 716 are not parallel or vertically inlinebut each sliding unit's distal end to be closest to the proximal end ofits neighboring strut (optimal position). Before the correction is tobegin in FIG. 30, the struts' lengths will be locked. To start thecorrection the software will determine the necessary movement of theshuttles units 726 to achieve proper alignment. Then, the sliding unitsor shuttles 726 move accordingly adjusting the position of the rings714, 716 until the deformity is corrected.

Alternatively, different length struts could be provided that “snap” into the system. This would allow the surgeon to get close to the optimalposition. As one decreases the iterative different in length of thestruts available, the probability of exactly attaining the optimalposition increases. As a corollary, this increases the number of strutsthat are to be offered in a kit and the complexity of the setup.Allowing for an adjustable length strut would reduce the number ofstruts required in a kit to six. It is important to note that theadjustable length does not in any way control the movement of the rings.Once the struts'lengths are set intraoperatively (while installing theframe), they are fixed for the entirety of the frame's movement. This isinherently different than prior art spatial frames because they requirethe struts to adjust length to make any movement. With the presentdesign, a user could adjust the struts 418 to the appropriate lengthbefore they were put on the frame, fix the lengths and install them intothe frame. This would insure that no strut length adjusting occurredwhile struts were on the frame.

The length of the struts needed is dependent on the initial position ofthe system 700. For example, if the system 700 is set up such that therings 714, 716 are parallel and vertically inline (See FIGS. 29 and 31),then all the struts 718 will be the same length. However, the distancebetween the rings 714, 716 dictates the required length of each strut718 to achieve the optimal position. In cases where the rings 714, 716are not parallel and vertically inline (See FIG. 30), the length of thestruts 418 is to be adjusted so the system 700 starts in the optimalposition. The following is a detailed description of the system.

FIGS. 26 and 27 illustrate an embodiment of a six strut externalfixation system generally denoted as 700. External fixation system 700includes an upper ring 714 and a lower ring 716. Each ring 714, 716includes a plurality of circumferentially extending teeth 715 and aplurality of bores 724 adjacent the inner diameter of the ring. Theexternal fixation system 700 includes a plurality of struts 718, whichcan be initially extended and then locked at a fixed length. Threestruts 718 are connected at a first end to a shuttle or sliding unit 726on ring 714 by a universal joint 728 and at a second end to one of theholes 724 in ring 716 via a universal joint 729. The other three struts718 are connected to shuttle units 726 on ring 716 at one end by auniversal joint 728 and by a second universal joint 729 to a hole 724 onring 714.

FIG. 28 shows a sliding unit or shuttle 726 mounted on ring 714 and auniversal joint 728. Universal joint 728 is connected to shuttle unit726 and includes a yoke 701, which may be rotated about an axis 702 andtwo pin axis 704 and 706 respectively. In lieu of shuttle unit 726,external fixation system 700 may alternatively incorporate the shuttleunits 26 depicted in FIG. 2 or any other shuttle unit or mechanismsuitable for moving along the perimeter of a ring 714 or 716.

FIG. 29 shows rings 714 and 716 with three shuttles 726 mounted on eachring and six struts 718 respectively connected at a first end to ashuttle unit 726 and at a second end to a hole 724 on the opposite ring.Referring to FIG. 30, rings 714 and 716 are oriented at an angle to oneanother with the six struts 718 coupled at a first end to a shuttle 726on a first ring 714 or 716 and a second end to one of the holes 724 onthe other of the rings 714 or 716. As seen in FIG. 31, rings 714 and 716can be adjusted to a parallel orientation with respect to each other.Such adjustment can be achieved either by adjusting the initial lengthof each strut 718 and then manipulating the location of the six shuttles726 on their respective rings while maintaining the strut length fixedduring the manipulation. Of course, if the manipulation is the initialset-up, the struts 718 may be adjusted in length to ensure that therings 714, 716 are properly located with respect to a fractured longbone. Then movement of shuttle 726 will accomplish, over time, thedesired correction.

As shown in FIG. 32, rings 714 and 716 can be located in a secondangular position. To place rings 714 and 716 in the second angularposition, the respective shuttle units 726 are moved on the rings 714 or716 in a controlled manner. Again, the alignment can initially be set byadjusting the strut length. The location of the shuttle unit 726 on aring 714 or 716 can be controlled with a worm gear mechanism, as shownin FIG. 4F. Each shuttle unit 726 may contain a worm gear mechanism orany other suitable drive mechanism for facilitating movement of theshuttle unit 726 along a ring 714 or 716. The drive mechanisms arecoupled to a computer-controlled stepper or servo motors. Externalfixation system 700 further includes a controller or any other suitablemeans for implementing the various inputs to the stepper or servomotors. The controller includes a microprocessor capable of manipulatingdata according to a mathematical equation describing the movement and/orposition of the rings 714, 716.

The following mathematical expressions describe the movement andposition of rings 714 and 716. This mathematical representation of a sixstrut platform is described by the position of one platform relative tothe other.

FIG. 33 shows one of the six kinematic chains connected to a base and amoving platform in the proposed parallel robot configuration. Eachplatform is a ring with a known radius. Each kinematic chain has anendpoint on the base and moving platform. The endpoints' positions aboutthe perimeter of each platform are measurable and therefore known. Asseen in FIG. 34, the center points shall be described as point O (centerof base with radius a) and point C (center of moving ring with radiusb). As the strut is connected to the base and moving platform theposition can be defined in a cartesian coordinate system. Point A isdescribed in the coordinate system of the base, which is (x,y,z) andpoint B is described in the coordinate system of the moving platformwhich is (x′,y′,z′). The struts' lengths are known and the distancebetween A and B is further described as ρ.

The rotation of the moving platform relative to the base platform can beexpressed as the rotation of the (x′,y′,z′)-coordinate system relativeto the origin coordinate-system (x,y,z). The angles for the rotation areset to (Ψ,Φ,Θ).

The vector {right arrow over (OC)} describes the position of the centerof the moving platform relative to the base platform and is representedby{right arrow over (OC)}={right arrow over (OA)}+{right arrow over(AB)}+{right arrow over (BC)}  (1)This equals to|{right arrow over (OC)}|=|{right arrow over (OA)}+{right arrow over(AB)}+{right arrow over (BC)}|  (2)

Equation (2) squared delivers an equation in which most of thecomponents can be replaced by known variables.∥{right arrow over (OC)}∥ ² =∥{right arrow over (OA)}∥ ² +∥{right arrowover (AB)}∥ ² +∥{right arrow over (BC)}∥ ²+2(∥{right arrow over(OA)}∥∥{right arrow over (AB)}∥+∥{right arrow over (AB)}∥∥{right arrowover (BC)}∥+∥{right arrow over (BC)}∥∥{right arrow over (OA)}∥)  (3)With ∥{right arrow over (AB)}∥=ρ and ∥{right arrow over (OA)}∥=α

The vector {right arrow over (BC)} is described in the origin basecoordinate system and equals its description in the rotated coordinatesystem x′,y′,z′ when it's multiplied with the rotation matrix R.({right arrow over (BC)})_((x,y,z)) =R({right arrow over(BC)})_((x′,y′,z′))  (4)

The rotation matrix rotates each axis with the according angle (Ψ,Φ,Θ)).R=[R _(z)(ψ)·R _(y)(Θ)·P _(x)(Φ)]  (5)

This results in the following rotation matrix:

$\begin{matrix}{R = \begin{bmatrix}{\cos\;{\Theta cos\psi}} & \begin{matrix}{{\sin\;{\theta sin\phi cos\psi}} +} \\{\cos\;{\phi sin\psi}}\end{matrix} & {{{- \cos}\;{\phi sin\theta cos\psi}} + {\sin\;\psi}} \\{{- \cos}\;{\Theta cos\psi}} & {- \begin{matrix}{{\sin\;{\theta sin\phi cos\psi}} +} \\{\cos\;{\phi sin\psi}}\end{matrix}} & {{\cos\;{\phi sin\theta cos\psi}} + {\sin\;\psi}} \\{\sin\;\theta} & {{- \sin}\;{\phi cos\theta}} & {\cos\;{\phi cos\theta}}\end{bmatrix}} & (6)\end{matrix}$Substituting all known variables into equation (3) gives us:∥{right arrow over (OC)}∥ ²=α²+ρ² +∥{right arrow over(BC)}+2[αρ+ρ(R{right arrow over (BC)})+α(R{right arrow over (BC)})]  (7)

Given six unique struts, we have six unknowns: (x′,y′,z′) which arecoordinates of the point C and (Ψ,Φ,Θ) which are the angles of rotationof the normal to the circle with the center C. Equation (7) would giveus 6 equations for the 6 unknowns from which the location of C can becalculated. Center C and radius b can be used to describe the positionof the moving platform relative to the base platform.

With reference to FIG. 35, the description of the ring of the movingplatform with respect to the center C of the base platform is a vectorconnecting Point 0 and point.{right arrow over (OB)}={right arrow over (OC)}+{right arrow over(CB)}  (8)

With the known rotation of the coordinate system, the circle lies in aplane spanned by the x′- and y′-axes. The normal to this plane is thez′-axes. The radius b rotates around this normal with the angle t.Therefore the circle is described by equation (9):{right arrow over (OB)}(t)={right arrow over (OC)}+b cos(t)x′+bsin(t)y′  (10)Using the rotation matrix derived from the equations above, we get theequation for a ring with respect to the origin coordinate system(x,y,z):{right arrow over (OB)}(t)={right arrow over (OC)}+b cos(t)Rx+bsin(t)Ry  (10)

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. An external fixation system comprising: first and second ringelements, the first ring including a gear track around a circumferencethereof; three shuttles mounted on the first ring element for controlledmovement about a circumference of the first ring element each shuttlehaving a first connector including a joint having three degrees offreedom and the system having three second connectors fixed to thesecond ring, the three shuttles having a gear engaging the gear track onthe first ring; three kinematic chains each having first ends connectedto a respective shuttle by the first connector and a second endconnected to a respective second connector, the second connectornon-rotatably mounted on the second ring, the second connector having auniversal joint freely rotatable about a first axis parallel to thesecond ring element and rotatable in a controlled manner about a secondaxis perpendicular to the first axis; and drive means for controllingthe movement of each shuttle about the circumference of the first ringand for controlling the movement of the kinematic chain second end aboutthe second axis of the second connector on the second ring.
 2. Theexternal fixation system as set forth in claim 1 wherein the jointhaving three degrees of freedom is a spherical joint.
 3. The externalfixation system of claim 1 wherein the drive means for controlling theposition of the shuttle comprises a computer controlled drive motor forrotating the shuttle gear.
 4. An external fixation system comprising:first and second planar at least part-circular ring elements, centers ofthe first and second ring elements spaced along an axis, the first ringelement having a circumferential gear track extending along thepart-circular circumference thereof; three and only three struts eachhaving a first and second end, the first end of each strut coupled tothe first ring by a first connector and the second end of each strutcoupled to the second ring by a second connector; the second connectorat the second end of each strut having a universal joint allowingrotation about a first axis parallel to the plane of the second ring androtation about a second axis perpendicular to the first axis; the firstconnector including a shuttle mounted on the track of the first ring formovement therealong and a joint having three degrees of freedom coupledto the first end of each strut, the shuttle having a gear engaging thegear track on the first ring; and drive means for controlling theangular position of each strut second end about the second axis anddrive means for rotating the shuttle gear for controlling the positionof each shuttle along the circumferential gear track on the first ring.5. The external fixation system as set forth in claim 4 wherein thefirst and second rings are complete circles.
 6. The external fixationsystem as set forth in claim 5 wherein the shuttles are spaced atpredetermined locations around a circumference of the first ring.
 7. Theexternal fixation system as set forth in claim 6 wherein each shuttlecan move along the track on the ring in predetermined increments.
 8. Theexternal fixation system as set forth in claim 4 wherein the struts areof variable length and include means for locking the strut at a desiredlength.
 9. The external fixation system as set forth in claim 4 whereinthe struts are of a fixed length.
 10. The external fixation system asset forth in claim 4 wherein the joint having three degrees of freedomis a spherical joint.
 11. The external fixation system of claim 4wherein the drive means for controlling the position of the shuttlecomprises a computer controlled drive motor for rotating the shuttlegear.